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  <a href="https://github.com/Viglino/ol-ext" class="icss-github-corner"><i></i></a>

  <a href="../../index.html">
    <h1>ol-ext: Fast edge detection filter</h1>
  </a>
  <div class="info">
    <p>
      SVG filter comes with a set of fast operators to detect edge on map layers.
      <br/>
      Filter: <select onchange="setFilter(this.value);">
        <option value="laplacian-8">Laplacian-8</option>
        <option value="laplacian-4">Laplacian-4</option>
        <option value="prewitt">Prewitt</option>
        <option value="roberts">Roberts cross</option>
        <option value="sobel">Sobel like</option>
      </select>
    </p>
    <ul>
      <li class='laplacian-8 laplacian-4'>
        <i>ol/ext/SVGFilter/Laplacian</i> is a <a href="https://en.wikipedia.org/wiki/Laplace_operator">simple edge detector</a> 
        filter to detect edges on images.
        <p>
          The Laplacian operator uses a single 3×3 kernel to detect edges.
        </p>
<pre class="table">
-1 -1 -1
-1  8 -1
-1 -1 -1
</pre>
or
<pre class="table">
 0 -1  0
-1  4 -1
 0 -1  0
</pre>
      </li>
      <li class='prewitt'>
        <i>ol/ext/SVGFilter/Prewitt</i> is a <a href="https://en.wikipedia.org/wiki/Prewitt_operator">Prewitt like operator</a>.
        <p>
          the operator uses two 3×3 kernels which are convolved with the original image to calculate 
          approximations of the derivatives - one for horizontal changes, and one for vertical.
        </p>
<pre class="table">
+1  0 -1
+1  0 -1
+1  0 -1
</pre>
&
<pre class="table">
+1 +1 +1
 0  0  0
-1 -1 -1
</pre>
      </li>
      <li class='roberts'>
        <i>ol/ext/SVGFilter/Roberts</i> is <a href="https://en.wikipedia.org/wiki/Roberts_cross">Roberts cross like operator</a>.
        <p>
          the operator the following two kernels.
        </p>
<pre class="table">
+1  0  0
 0  0  0
 0  0 -1
</pre>
&
<pre class="table">
 0  0 +1
 0  0  0
-1  0  0
</pre>
      </li>
      <li class='sobel'>
        <i>ol/ext/SVGFilter/Sobel</i> is <a href="https://en.wikipedia.org/wiki/Sobel_operator">Sobel like operator</a>.
        <p>
          The operator uses two 3×3 kernels which are convolved with the original image to calculate 
          approximations of the derivatives – one for horizontal changes, and one for vertical
        </p>
<pre class="table">
+1  0 -1
+2  0 -2
+1  0 -1
</pre>
&
<pre class="table">
+1 +2 +1
 0  0  0
-1 -2 -1
</pre>
      </li>
    </ul>
    @See: <a href="./SVGFilterComparison.html">SVG filter comparison</a>
  </div>

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    var photo = new ol.layer.Geoportail({ layer: 'ORTHOIMAGERY.ORTHOPHOTOS', className: 'photo', opacity: .4 });
    var plan = new ol.layer.Geoportail({ layer: 'GEOGRAPHICALGRIDSYSTEMS.PLANIGNV2', className: 'photo' });
    var layer = new ol.layer.Geoportail({ layer: 'ORTHOIMAGERY.ORTHOPHOTOS', className: 'edge', title: 'Edges', opacity: 1 });
    // Load source only once
    layer.setSource(photo.getSource())

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    var map = new ol.Map ({
      target: 'map',
      view: new ol.View ({
        zoom: 18,
        maxZoom: 19,
        center: [343183, 6554838]
      }),
      layers: [ photo, /* plan, */ layer ]
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      "laplacian-8": new ol.filter.SVGFilter(new ol.ext.SVGFilter.Laplacian({ alpha: true })),
      "laplacian-4": new ol.filter.SVGFilter(new ol.ext.SVGFilter.Laplacian({ neighbours: 4, alpha: true, gamma: 2 })),
      prewitt: new ol.filter.SVGFilter(new ol.ext.SVGFilter.Prewitt({ alpha: true })),
      roberts: new ol.filter.SVGFilter(new ol.ext.SVGFilter.Roberts({ alpha: true })),
      sobel: new ol.filter.SVGFilter(new ol.ext.SVGFilter.Sobel({ alpha: true }))
    }

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    }
    setFilter('laplacian-8');


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      layer: layer
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